TABLE OF COMMON PERMANENT MAGNET MATERIALS:
MAGNETIC TERMS and DEFINITIONS
Most magnet design engineers are familiar with the widely used Alnico materials: Alnico
V, VI, VIII, and IX which require magnetising
forces ranging from 3000 to 7000 oersteds, with energy products of the order of 7.5 million gauss-oersteds, as
well as the barium ferrites,
which require a magnetising force of 12,000 oersteds. These presented some problems in magnetising, initially, but are now used
in
many production applications. Recent research on the so-called rare earths, which include Cerium, Lanthanum, Lutetium, Platinum,
Neodymium and Samarium,
has presented the design engineer with many new magnet materials, plus a new set of problems in their
utilisation. Typical properties are given below of some
of these materials now available in production quantities. They include energy
product values of 14 million gauss-oersteds and required magnetising forces of
20,000 to 60,000 oersteds. Theoretical limits are said to
be on the order of 45 million gauss-oersteds, which would require magnetising forces of 120,000
oersteds.
The advent of these new materials permits the design engineer to undertake magnetic
assembly designs, which were theoretically,
and economically impossible several years ago. The engineer must also thoroughly evaluate his design to be certain
that once the
assembly is fabricated, it can be magnetised. This is especially true of multi-pole structures. In many cases the magnet can be charged
prior to
installation in an assembly. However, due to ferrous contamination possibilities, physical handling difficulties, and similar
drawbacks, may preclude the
possibility of such "pre-magnetisation".
The higher magnetising force requirements also necessitate that existing magnetising
fixtures be re-designed and, in some cases,
the magnetising equipment, in order to achieve the forces required. With materials having coercive forces greater
than 10.000 oersteds
it is generally not necessary to stabilise a structure to prevent inadvertent change of flux density levels, although treating will be
required in some instances, to set or calibrate the level of flux density required. Here the potential high energy of the stored energy
treater will bean
added asset.
MAGNET TERMS AND DEFINITIONS
In discussing the factors, which must be considered in choosing the type of magnet,
charging and stabilising equipment, certain magnetic
terms will be referred to. An understanding of these terms will prove helpful. Magnetic terms are
generally defined in either of the two
types of systems, the SI system, and the cgs (centimetre-gram-second) system. All terms in this publication are
defined in the cgs system.
Anisotropic Magnetic Material
Also called oriented material. An anisotropic magnet has a preferred direction of
magnetisation. To realise the maximum potentialities
from such materials, they must be magnetised along the preferred axis. Orientation is accomplished by
means of an applied magnetic
field during manufacture of the material. Anisotropic materials do not have a preferred polarity orientation; i.e. either of the
poles may
be north or south.
Coercive Force
This is the magnetising force, in oersteds, which must be applied to a magnetic
material, in a direction opposing the residual induction,
to reduce the induction to zero. It may be considered
the criterion that determines the ability of a magnet to resist demagnetising
influences. A material having a high coercive force is more difficult to
demagnetise than a material having a low value of coercive force.
Diamagnetic Materials
This describes materials that have permeability slightly less than one. These materials
tend to be slightly repelled by a magnetic field.
Ferromagnetic Materials
These materials have characteristics similar to that of iron. (Having a high degree of permeability.
)
Isotropic Magnetic Materials
These materials do not have a preferred axis of
magnetisation.
Magnetic Field Intensity
The strength, of a magnetic field, in air, measured at any point in a magnetic circuit, It can be measured as either oersteds or gauss’s,
since, in air, B (gauss’s) numerically equals H (oersteds)in the
cgs system. Thus, if we say the field in a magnetic air gap is 3000 oersteds,
we are also correct in saying the same field equals 3000 gauss’s. It must be
remembered, however, that B and H are two distinctly
different physical phenomena, and that the numerical equality exists only in air.
Magnetising Force
This describes the magnetomotive force (force, which tends to produce a magnetic field)
per unit length. Symbolised as H, the unit is
called the oersted. For example: Alnico V magnet material requires a magnetising force of 3000 oersteds for
saturation. In reference
to magnet chargers, it is the magnetising force developed by the charging fixture, either in the air gap between charging poles, in
air
surrounding a magnetising conductor, or in the cavity of a solenoid-charging fixture.
Magnetic Induction
Symbolised as B (gauss’s) and is the magnetic flux per unit of a magnetic section
perpendicular to the direction of flux. An interesting
point to note is that if a magnet material (Alnico V for example) is placed in a magnetic field (assume
3000 oersteds in this case) the flux
density in the magnet will instantaneously rise to a value B (15000 gauss’s for Alnico V), far in excess of the
magnetic field in air.
The relationship of B and H in air therefore no longer holds, since B is measured in a magnet material having permeability greater
than
the permeability of air.
Paramagnetic Materials
These are materials that have “a permeability” only slightly greater than one,
usually between 1.000 and 1.001. When ferromagnetic
substances are heated to a temperature above their Curie point they become paramagnetic, until their
temperature is reduced to
below the critical point. Such materials are said as being “feebly attracted by a magnetic field”.
Reluctance
Generally speaking, reluctance is a measure of the ability or inability of a material to
transmit or carry a magnetic field. Air may
be considered a high reluctance path and soft iron a path of low reluctance.
Residual Induction
The magnetic induction remaining in a permanent magnet after the magnetising force is
removed. It is measured in gauss’s.
Retrace-ability
Generally refers to the ability of a magnet assembly to have a predictable flux density
at any given temperature within certain
limits. For example: Assume a magnet assembly to have a flux density of 2000 gauss’s at 250C after magnetic
stabilisation. The
structure is then subjected to temperature extremes of 0 deg. C to 1000 degree C.
After the temperature cycling, the assembly
may have a measured flux density of 1980 gauss’s at 250C. However, any further temperature cycling, providing
the limits of 0 C
and 100 C are not exceeded, will not affect the flux density at 25 degree C, i.e., 1980 gauss’s. The flux density' at any temperature
between the cycling extremes, will also be retraceable. For Alnico V the flux density usually drops 0.02% per degree C rise above
a specified temperature, and
conversely exhibits a 0.02% per degree C increase, with lowering temperature.
Saturation
Describes the condition of a magnet when it is as fully magnetised as possible. To
realise the full stability potential of a magnet it
should always be saturated during charging, even though some demagnetisation may be necessary for the
proper operation in
the final magnetic assembly.
Stabilisation
Reducing the residual induction in a magnet to a level where it will not be affected by
any demagnetising forces that may be
encountered during normal operation of the finished magnetic assembly. Often called artificial aging. This is generally
accomplished
by subjecting the magnet assembly to an alternating magnetic field of adjustable intensity until the required flux density is
reached in the
magnetic air gap. Temperature stabilisation is accomplished by subjecting the magnet assembly to high and
low temperature cycles, simulating the maximum
temperature extremes that will be encountered by the assembly in its
operating environment. Temperature cycling does not prevent changes in flux density with
change of temperature, but it
does allow operation of the structure with good retrace-ability characteristics.
CONVERSION FACTORS
Symbol CGS Unit SI Unit Conversion of CGS to SI
Induction (B) Gauss weber/meter2 x 10~-4
Induction (B) Gauss Tesla (T) . x 10~-4
Magnetic Field Intensity (H) Oersted ampere-turn/meter
x79.58
Permeability (U) Gauss/oersted Henry/meter x 12.56 x 10
~-6
Induction flux (Q) Maxwell (line) Weber x 10~-8
Magnetomotive force (MMF) Gilbert Ampere-turn x 79.55 x
10~-2
Reluctance (R) Gilbert/Maxwell Amp-tum/Weber x 79.55 x
10~-3
Permeance (P) Maxwell/Gilbert Weber/amp-turn x 12.56 x
10~-9
'Note: Induction is designated as Tesla in the internationally established SI system.
TABLE OF COMMON PERMANENT MAGNET MATERIALS:
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